Ion trap mass spectrometers (hereafter called ion traps) are well known devices for receiving and analyzing ions. Typical ion traps are shown in U.S. Pat. Nos. 4,736,101 issued Apr. 5, 1988 and 4,540,884 issued Sep. 10, 1985, both to Finnigan Corporation.
Ion traps typically employ a ring electrode and end caps which, when suitable RF and DC voltages are applied to them, provide a quadrupole field to trap ions within a storage region. However ion traps are usually relatively small in physical size and have the capacity to store only a limited number of ions. When the number of ions injected into an ion trap becomes too large, space charge effects occur which have a number of undesirable consequences. These consequences can include spontaneous emptying of the trap, shift in the mass calibration, distortion of the analysis results obtained from the ion trap, and the like.
In addition, when an ion trap is performing an analysis, it cannot accept additional ions. If a prolific ion source is used, the time taken to fill the ion trap can be much less than the time required for the ion trap to perform analysis. During the analysis time, the ions produced by the ion source may be wasted, resulting in a very low duty cycle for the ion trap and causing low sensitivity for the system.